Hugo Pacheco:
I have recently tried to replicate some examples from in the
articles about type families but found some possible bugs.
In [2], the example
class C a where
type S a (k :: * -> *) :: *
instance C [a] where
type S [a] k = (a,k a)
does not compile under the claim that the type variable k is not in
scope.
However, if we extract the type family from the class
type family S a (k :: * -> *) :: *
type instance S [a] k = (a, k a)
class C a
it compiles correctly.
According to [3], the difference is purely syntactic sugar, does
that mean that both examples should compile and behave the same or
is there some subtlety that justifies the class example not to
compile?
I am sorry for the confusion this causes, but we wrote the paper
before the implementation in GHC was finalised. Hence, the
implementation deviates from the paper in some aspects. Generally,
please use
http://haskell.org/haskellwiki/GHC/Type_families
(which will be integrated into GHC's Users Manual for 6.10) as the
definite guide to the implementation.
Here a brief rational for this change in design. We originally
intended to distinguish between type parameters that do use type
indexing (the a in (S a k)) and those that do not (the k in (S a
k)). However, we found later that this leads to implementation
problems. The new rule is that any explicitly named parmeters, eg, s
and k in
type family S a (k :: * -> *) :: *
are indexed. If you don't want to use the second parameter as an
index, write
type S a :: (* -> *) -> *
This is also simpler to explain, as simply *all* explicitly named
parameters in a type family declaration are indicies.
Another issue is that data kinds (used in both [2] and [3]) do not
seem to be supported at all by the compiler, are they already
implemented in GHC?
Simple examples such as
datakind Nat = Zero
or
datakind Nat = Zero | Succ Nat
fail to compile.
No, datakinds aren't supported yet. We still intend to add them, but
our first priority is to thoroughly debug the existing functionality
and especially to make sure the interaction with GADTs works well.
Thanks for the feedback.
Manuel
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